Add a Download button that exports a database target's archive as gzipped JSON (closes #374)

Each database target on the webhook page has a Download button that streams its archive as gzipped JSON, archive-WEBHOOKNAME-TARGETNAME-TIME.json.gz, with names made safe by delivery.ArchiveFileName's function. The export reads one consistent snapshot through one cursor in a read-only transaction, so archive writes carry on, and holds the rename lock only while it reads the stored names and opens the file. It extends its write deadline as it writes, so a large archive downloads for as long as the client reads; a failure after the response has started aborts the connection so the browser marks the download failed. The request limit is now the service's own middleware, which no longer writes a 504 over a response already started.

Model: opus-5-5

internal/handlers/handlers.go

@@ -97,7 +97,8 @@ type Handlers struct {
 	// names through the archive rename, the save and any move back.
 	// Interleaved, one could rename an archive between another's
 	// rename and save, leaving the file named for one edit and the
-	// stored names from the other.
+	// stored names from the other. An archive download holds it while
+	// it reads the stored names and opens the file they give.
 	renameMu sync.Mutex
 
 	// dummyVerifications counts the equivalent-cost verifications